1. Field of the Invention
The present invention relates to a method of diffusing an impurity during the manufacturing of a semiconductor device in particular, to a method of diffusing an impurity having a specified conductivity type, from the layer containing the impurity formed on a semiconductor substrate into the substrate.
Diffusion methods of this type, such as a method of using the insulating film containing an impurity and a method of using the semiconductor layer containing an impurity are generally known.
2. Description of the Prior Art
FIGS. 1A to 1E show the method of diffusing an impurity where phospho-silicate-glass (PSG) film is used as an insulating film containing an impurity.
In FIG. 1A, reference numeral 1 indicates a p type silicon semiconductor substrate, while reference numeral 2 is a silicon dioxide (SiO.sub.2) film.
The SiO.sub.2 film 2 is provided on the substrate with a window at a specified area.
In FIG. 1B, reference numeral 3 indicates a PSG film. This PSG film 3 contains about 20% phosphorus and has a thickness of about 1 .mu.m.
FIG. 1C illustrates a semiconductor substrate 1 after being annealed for about 20 minutes at a temperature of about 1050.degree. C. in the nitrogen (N.sub.2) atmosphere. Thereby, the n type impurity diffused portion shown, reference numeral 5 is formed. The depth of the portion 5 is about 0.4 .mu.m and its sheet resistance .rho..sub.s is about 20 .OMEGA./square.
To make electrical connection with this impurity diffused portion 5, a window 6 is formed as shown in FIG. 1D.
As shown in FIG. 1E, an aluminum layer (Al) 7 is ordinarily used as the contact electrode. On the occasion of obtaining an ohmic-contact between the impurity diffused portion 5 and the Al layer 7 provided on the semiconductor substrate, the formation of an alloy of the Al and Si sometimes proceeds abnormally, thus destroying the shallow pn junction formed in the semiconductor substrate 1. Therefore, a thin polycrystalline silicon is formed on the silicon substrate surface, and then after forming the Al layer 7 thereon, the Al and Si are alloyed. Usually, the thermal process for rounding the edges of the window of the PSG layer 3 is carried out before depositing the Al, in order to prevent breaks in the Al layer 7.
As explained above, the impurity diffusion method utilizing the insulation film containing an impurity requires various processes, and also requires a variety of steps for forming a low resistance electrode contacting the impurity diffused area formed in the semiconductor substrate.
FIGS. 2A through 2C show an impurity diffusion method where polycrystalline silicon is used as the semiconductor layer containing an impurity.
In FIG. 2A, reference numeral 11 indicates a p type silicon semiconductor substrate, reference numeral 12 designates a silicon dioxide film and reference numeral 17 designates a polycrystalline silicon layer.
The thickness of polycrystalline silicon (poly-Si) layer 17 is about 0.4 .mu.m. The poly-Si layer may contain an impurity when formed or be doped with an impurity after a non-doped poly-Si is formed.
Thereafter, the annealing is carried out for about 20 minutes at a temperature of about 1050.degree. C. in a N.sub.2 atmosphere. Thereby, the n type impurity diffused portion indicated by the reference numeral 15 in FIG. 2B is formed. The depth of the impurity diffused portion 15 at the center is about 0.4 .mu.m, but the depth at the portion around the end of SiO.sub.2 window is about 0.7 to 0.8 .mu.m as indicated by the reference numeral 18 in FIG. 2B.
This deep impurity diffused portion 18 is usually called the "Dragon teeth" and is considered to be generated by causes explained hereunder.
(i) A stress concentration occurs inside the silicon substrate 1 due to the difference of expansion coefficients of the SiO.sub.2 film 12 and poly-Si layer 17. This stress concentration allows an impurity to abnormally diffuse into the lattice defect generated by the stress.
(ii) The phosphorus contained in the poly-Si layer 17 extending over the SiO.sub.2 film 12 is concentrated in the portion around the end of window and then diffused thereto.
However, such deep diffused portion is not formed by the impurity diffusion process utilizing the PSG film indicated in FIG. 1. For this reason, the inventor of the present invention believes that the first reason (i) above is the major mechanism influencing the formation of "Dragon teeth."
In the impurity diffusion method utilizing poly-Si, it is possible to execute the diffusion process, thereafter pattern the remaining poly-Si layer and use the poly-Si layer directly as the wirings. However, in the case of the ordinary poly-Si containing an impurity, the sheet resistance is as large as 50 to 70 .OMEGA./square.
When the poly-Si layer is used as the impurity diffusion source, the contact electrode can be formed rather easily from the impurity diffused portion and the process is also simplified.
In FIG. 2C, reference numeral 19 indicates the Al electrode. To obtain an ohmic-contact between the electrode 19 and the n type impurity diffused portion 15 the poly-Si layer 17 used for the impurity diffusion and the Al layer 19 must be alloyed.
As explained previously, it is possible in the process for diffusing impurity from the layer containing impurities, to form the shallow diffusion portion in a small area. However, it is difficult to form a contact electrode without deteriorating the element characteristics by abnormal diffusion in a part of the small diffusion area.
For this reason, a novel impurity diffusion method is required which solves problems of the abovementioned conventional technique.